An example of a known displacement sensor is illustrated in FIG. 7.
The illustrated known displacement sensor 300 includes a body 310 mounted to a mechanical device D1, an input bar 320 in the form of a cantilever integrally extending from the body 310, a distal end of which comes into contact with a partial region D2 of the mechanical device D1 so as to undergo displacement according to displacement of the partial region D2, a strain gauge 331 attached to the input bar 320, and an electric circuit board 332 to generate an electrical signal upon receiving a strain value measured by the strain gauge 331.
When the partial region D2 is moved upward, the input bar 320 is bent upward thus undergoing displacement. The strain gauge 331 and the electric circuit board 332 generate an electrical signal corresponding to the displacement of the input bar 320.
In the above described known displacement sensor 300, the strain gauge 331 is attached to the input bar 320 to attain a sufficient sensitivity. In addition, to prevent contamination due to impurities under specific use environments, a cover 340 is provided to maintain the strain gauge 331 and the electric circuit board 332 in an airtight condition.
In this case, to maintain the strain gauge 331 in an airtight condition, it is necessary to attach a part of the cover 340 to the input bar 320. However, this causes the cover 340 to undergo displacement along with the input bar 320, having an effect on a measured value of the strain gauge 331.
Moreover, the cover 340 may be unintentionally detached from the input bar 320 due to frequent deformation of the input bar 320.
FIG. 8 is a partial sectional view illustrating a known load sensor using a diaphragm, to which a strain gauge is attached.
The known load sensor 200 includes a sensing body 210, a diaphragm 211 positioned on the sensing body 210, and an input bar 212 positioned beneath the sensing body 210 to vertically press the diaphragm 211 by a weight thereof.
A strain gauge 213 is attached to the diaphragm 211. The strain gauge 213 includes a pair of first and second piezo-resistance elements R1 and R2 attached close to one side edge of the diaphragm 211, and a pair of third and fourth piezo-resistance elements R3 and R4 attached close to an opposite side edge of the diaphragm 211 so as to correspond respectively to the first and second piezo-resistance elements R1 and R2.
A notch 214 is indented in a lower surface of the sensing body 210 and acts to increase strain of the diaphragm 211.
Assuming that the input bar 212 is fixed at or comes into contact with a load occurrence position of a target device to be measured, the input bar 212 is mainly subjected to vertical force. In addition to the vertical force, the input bar 212 is typically subjected to horizontal force, twisting moment, etc, thus being under the influence of miscellaneous load including moment, torsion, etc.
When attempting to detect deformation of the diaphragm 211 caused by the vertical force to be measured using the strain gauge 213, deformation of the diaphragm 211 due to the miscellaneous load including moment, etc. may be detected simultaneously. Therefore, it is necessary to eliminate the miscellaneous load in order to measure only the vertical force.
Accordingly, in a known solution, as shown in FIG. 9, a Wheatstone bridge circuit consisting of first to fourth piezo-resistance elements is used. The Wheatstone bridge circuit is configured such that strains measured by the piezo-resistance elements under the influence of moment offset each other and only deformation of the diaphragm caused by vertical force can be measured.
In the meantime, in the case where the known load sensor is used as a displacement sensor, it is necessary for the diaphragm to be oriented orthogonal to a surface of a mechanical device that undergoes displacement. This disadvantageously results in a limited installation position of the sensor.
In particular, if a possible installation space of the displacement sensor is limited, for example, if it is difficult, in the case of measurement of displacement of a vehicular electronic brake caliper, to attain a space required for the displacement sensor to be orthogonally attached to a displacement occurrence surface, the use of the displacement sensor may be impossible.
With relation to design of a sensor, it is important to provide the sensor with not only high strength, but also sufficient strain for stable amplification in a circuit. However, the sufficient strain and the high strength are conflicting characteristics from various viewpoints and thus, design trade-off is necessary. For this reason, when a displacement sensor is designed based on the conception of a load sensor that is adapted to receive force directly, the displacement sensor may entail a problematic strength, resulting in vulnerable sensor design. Accordingly, to enable stable measurement of displacement regardless of a maximum operating load, it is necessary to design a displacement sensor such that the role of the displacement sensor is limited to accurately measure slight displacement of a specific region of a structure and a system operating load is assigned to the structure. This is a principal intent of the displacement sensor design. This is also advantageous for acquisition of a sensor installation space because it is unnecessary to arrange the sensor on a transmission path of force. Accordingly, upon design of the sensor, a system designer should consider only an operational displacement portion of the structure that can be measured by the sensor.
In the meantime, the above described known diaphragm has a significantly limited strain gauge attachment area. This results in troublesome attachment of the strain gauge and increases generation of defective products due to a deviated attachment position of the strain gauge.
More specifically, to accurately measure strain using the strain gauge, the strain gauge must be attached to a linearly deformable region of the diaphragm. In the case of the above described known diaphragm, only a partial region immediately above the notch undergoes approximate linear deformation and thus, the strain gauge must be accurately attached to the partial region. Therefore, despite use of an automated machine, there always exists a risk of a deviated attachment position of the strain gauge due to fine shaking and thus, generation of defective products may be increased.